The objective of this research proposal is to investigate and to clarify further the nature and possible roles of the enzyme(s) monoamine oxidase (MAO) in the heart. Results to date indicate the presence of multiple forms of MAO in the heart, and that much of it is localized in an extraneuronal site, i.e., on the outer wall of the mitochondria of cardiac muscle cells. Plans are presented (1) to investigate and to characterize the isozyme or multiple forms of the MAO by electrophoretic separation in normal and in chemically sympathectomized hearts; (2) to characterize the in vivo properties of extraneuronal MAO in the rat heart, and this includes gaining an understanding of how the MAO develops, continues to increase, and recovers after irreversible inhibition, and the factors which influence these processes; and (3) to determine the role of extraneuronal MAO in the heart (and eventually in other sympathetically innervated organs) in normal and pathological functions. The methods required of these studies include separation of purified MAO from various parts of the heart, employing polyacrylamide gel electrophoresis and several substrate systems. Heart slices and perfused hearts after chemical denervation by 6-hydroxydopamine administration will be employed for determining extraneuronal uptake mechanisms and deamination of perfused H3-NE and H3-DA. The role of MAO in innervated vs. denervated hearts on the genesis of isoproterenol induced cardiomyopathy will be investigated in appropriately treated animals and in animals whose cardiac MAO activities are partiaaly inhibited. The reason these objectives have been formulated is that more and more evidence is accumulating for the presence of substantial extraneuronal sources of MAO in most tissues, but until recently little attention has been given to it. The function of intraneuronal MAO in noradrenergic neurons is well accepted, but with the evidence of extraneuronal uptake of MAO it is necessary to establish the functional role of this enzyme. Moreover, an analysis of this relationship may clarify many of the questions raised by the MAO inhibitors and may further our understanding of the role(s) of the MAO(s) in adrenergic neurotransmission.